powerpc/mm: Fix typo of cpumask_clear_cpu()

The function name of cpumask_clear_cpu was not correct. Fortunately
nobody uses that code with hotplug yet :-)

Reported-by: Jin Qing <b24347@freescale.com>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>